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Mystery deepens over origin of biggest black holes

By David Shiga

Monstrous black holes have been found at the centres of galaxies only a few hundred million years after the big bang – how they got so big so fast is one of the biggest puzzles in astronomy (Illustration: NASA/JPL-Caltech)

Where did the universe’s biggest black holes come from? One idea suggests the behemoths began as smaller “seed” black holes that gobbled up surrounding gas. But new computer simulations suggest these seeds were born with practically nothing around them to eat, deepening the puzzle over how the biggest black holes came to be.

Quasars are extremely luminous objects, thought to be sites where gas heats up and glows brilliantly as it plunges into a black hole with a billion or more times the mass of the Sun. They have been spotted at vast distances from Earth – so far that they appear as they were less than a billion years after the big bang.

How these supermassive black holes grew so big so fast has been a major puzzle. Some astronomers have suggested that they grew from smaller black holes of about 100 times the Sun’s mass, left behind when the universe’s first stars collapsed at the end of their lives.

But the universe’s first stars were not born until a few hundred million years after the big bang. Even though they lived only a few million years before collapsing to form black holes, this does not leave much time for these seeds to grow into the monstrous black holes powering quasars.

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The puzzle now appears to have deepened, with new computer simulations suggesting that these seed black holes were born with little food around them from which to gain weight.

Tom Abel of Stanford University in California, US, and his colleagues made computer models simulating the first generation of stars. These first stars are thought to have been very massive and luminous, weighing about 300 times as much as the Sun. The simulations reveal that the stars’ prodigious radiation would have blown away the gas around them.

Early famine

As a result, the black holes that formed when the stars died a few million years later would have had very little to eat. In the simulations, it took about 100 million years for the gas to fall back towards the first black holes and provide them with something to eat. The time lost due to the early famine makes it even harder to imagine how these black holes could have swelled to billions of times the Sun’s mass soon thereafter.

“These are not progenitors for supermassive black holes as far as I can tell,” Abel reported last week at a conference at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US.

Volker Bromm of the University of Texas in Austin, US, agrees that the new work highlights a major problem for forming supermassive black holes via smaller ones.

An alternative that Bromm and others have investigated is the possibility that a massive gas cloud could collapse directly to form a black hole with between 1000 and 1 million times the mass of the Sun.

The powerful gravity of such a black hole would help it collect more gas, potentially allowing it to quickly grow to a billion solar masses. “That seems to be a promising idea,” Bromm told New Scientist.